World's increasing demand for energy and food can be met by use of non-edible agricultural produce like lignocellulosic biomass. Lignocellulosic biomass, e.g. as obtained from agricultural farm and industry residues, has proven to be a potential and sustainable resource as feedstock for production of sugars that are precursors for fuels, chemicals, feed and food products e.g. cellulosic ethanol, organic acids like lactic acid and succinic acid, cellulose, and food additives like xylitol.
Agricultural residues that can be used include rice straw, wheat straw, corn cob, corn stover, sugarcane bagasse, stevia leaves etc., while on the other hand lignocellulosic feedstocks may also be derived from forest products as well as by-products of agricultural industry. Commonly referred to as lignocellulosic biomass, or simply cellulosic biomass, the biomass constitutes an intricate complex of cellulose, hemicelluloses and lignin. Conversion of these substrates into sugars and further products involves three major steps; pretreatment, hydrolysis followed by chemical or biochemical transformation.
The pretreatment of the substrates can be designed so that it fractionates the substrate biomass into lignin and holocellulose (a mixture of cellulose and hemicellulose). While lignin can be used for the production of value added chemicals, the holocellulose components obtained can be hydrolysed using a combination of enzymes called cellulases for release of sugars. These sugars find uses for production of chemicals and materials through one or more combinations of chemical and biological transformations. The sugars, in addition to being precursors to fuels, energy and chemicals, can also find uses in food and pharmaceutical industry if isolated and purified to required levels.
Pretreatment is aimed at loosening the bonds between cellulose, hemicellulose and lignin. Acid pretreatment is the most widely used method but suffers from being non-ecofriendly and requires expensive material of construction besides giving lower yields of mono-sugars and forming fermentation toxic side products. Hydrothermal or steam explosion is the next popular choice but suffers from scalability problems. Alkali hydrolysis is expensive but produces high quality cellulosic residues and gives higher yields of fermentable sugars. Other options like AFEX, and solvent processes are not likely to find acceptance on account of the costs involved.
The choice of a method is decided by many factors, the most important being the type of biomass. Biomass is graded on the basis of severity of pre-treatment required to obtain enzyme hydrolysable biomass. Thus, while bagasse is a ‘low-severity’ biomass, wood-chips and cotton or jatropha plant waste are ‘high-severity’ biomass. Most agricultural grain residues would classify as low to medium severity biomass.
Most pre-treatment technologies leave residual solid mass that is ‘de-lignified’ and ‘softened’ for hydrolysis to sugars.
Option lies between sending the whole biomass through the next saccharification step without fractionation into separated components or sending the biomass into saccharification after separation of the fractionated components namely cellulose, hemicellulose and lignin. The earlier approach is normally preferred when all combined sugars i.e. glucose, xylose and others are to be together converted to ethanol. However, delignification is an important step as the products of lignin degradation e.g. phenolics, acetic acid, formic acid etc. are known to be inhibitory to the subsequent conversion processes.
Over the period of time, it has become apparent that separation of biomass into its constituent components namely, cellulose, hemicellulose and lignin has several advantages. These advantages include higher yields of sugars and lower consumption of enzyme in the saccharification step, especially when the objective is to make high purity sugars. Saccharification step constitutes the hydrolysis reactions whereby the polymeric cellulose and hemicellulose are broken down to their monomeric components. While many non-biological routes have been tried, it is by and large established that enzymatic hydrolysis can best provide sugars in high yields with lower formation of undesirable by-products.
There are, however, a number of issues that need attention in order to use the enzyme hydrolysis technology for commercial or scalable applications. The hydrolysis or enzymatic saccharification involves use of cellulases; most often a mixture of one or more each of endoglucanases, exoglucanases and glucosidases for breaking of cellulose polysaccharides into monomeric glucose moieties. It is a well documented fact that during the course of hydrolysis as glucose begins to accumulate in the system, glucosidase enzyme is inhibited leading to cellobiose accumulation. This causes inhibition of exoglucanases and endoglucanases with the overall result that the saccharification process stalls short of 100%. The result is a mixture of sugars consisting of monosaccharides like glucose, xylose, arabinose; disaccharide and oligosaccharides like cellobiose and cello-oligosaccharide, respectively.
Oligosaccharide and disaccharide concentrations differ depending on the reaction conditions and may vary from 0.1 mg/ml in low substrate loading reactions to 50 mg/ml where the substrate concentration is as high as 20%. Hydrolysis or separation of these oligosaccharides to monosaccharide is a prerequisite if high purity food grade glucose (more than 99.5% pure) is the desired product or even when the final product is used for conversions of glucose to other value added products.
There are several factors of concern in the economics of production of sugars by known methods for conversion of a cellulosic biomass to glucose. Since the demand of cellulose derived high purity glucose is likely to find increasing applications and acceptance in food industry and many other industries, a robust method capable of providing homogeneous and high purity glucose is therefore necessary.